Adding support for MAC_TASK_CONTEXTS and MAC_OBJ_CONTEXTS to userspace.

[Casey Schaufler]

On 6/14/2021 2:13 PM, Steve Grubb wrote:
> Hello,
>
> On Monday, June 14, 2021 3:34:33 PM EDT Casey Schaufler wrote:
>> I'm looking at the audit userspace implications of adding two
>> new kernel audit records. AUDIT_MAC_TASK_CONTEXTS and
>> AUDIT_MAC_OBJ_CONTEXTS are used when there are multiple security
>> modules with a "security context" active on the system. This
>> design has been discussed here at length. The records will look
>> like:
>>
>> 	AUDIT_MAC_TASK_CONTEXTS
>> 	subj_<lsmname>=value
>> 	subj_<lsmname>=value
>> 	...
>>
>> Looking at the audit user-space code I see several things
>> that have me concerned. The first is the use of WITH_APPARMOR.
>> Going forward what behavior would we want if subj_apparmor=something
>> shows up on a system that has not got WITH_APPARMOR defined?
> I think it should be ignored.
>
>> The code is inconsistent in that it does not use WITH_SELINUX,
>> but that's hardly a surprise given its origins. There is also no
>> WITH_SMACK, but that's unlikely to be an issue since Smack's use
>> of audit is very much like SELinux's.
> We can add those WITH_* if you like.
>
>> The question is what to
>> do about filtering when subj=foo is specified. I suggest that if
>> any of subj_selinux, subj_smack or subj_something is "foo", it is
>> a match.
> I think that's how we already treat things. There is a linked list for AVC's 
> and we match on any of.
>
>> But the SELinux components of a label (level, user, ...)
>> are also available for filtering. If someone wrote a simple Bell &
>> LaPadula LSM filtering by some of those fields could be useful
>> there, too.
>>
>> I would like guidance on whether I ought to go the route of
>> more extensive use of WITH_APPARMOR (and WITH_SMACK, WITH_MUMBLE)
>> or take the path of greater generalization. Or, whether I should
>> treat each case individually and give it my best whack.
> To be honest, I have no idea how well the audit system works with any MAC 
> system except SE Linux.

Understood. Part of what I'm looking at is ensuring that as multiple
concurrent LSMs come in that the audit user-space isn't mucked up.
ausearch has these options:

	-o,--object <SE Linux Object context>
	-se,--context <SE Linux context>
	-su,--subject <SE Linux context>

Without multiple LSMs we can easily ignore "SE Linux" in these
options and use whatever kind of "context" is available. If I
have SELinux and AppArmor, the implication is that you can't
search on AppArmor information. Should we be adding

	-aa,--apparmorcontext <AppArmor context>
	-as,--apparmorsubject <AppArmor subject context>

or should we change -se to look at all "contexts", and change
the description to reflect that? Basicaly, I'm asking whether you'd
rather add options for other LSMs or remove descriptions that
specify SELinux.

>  I don't really know if its doing the right thing. 
> Ausearch and report share a parser. It is time sensitive. I usually test it 
> on 4 or 5 Gb of logs. We also have the ausearch-test program which can be 
> used to test any changes to the parser.
>
> http://people.redhat.com/sgrubb/audit/ausearch-test-0.6.tar.gz
>
> Once that is squared away, there is the auparse library. It has a table that 
> classifies a field name into what it is for interpretation purposes. You will 
> find a #ifdef WITH_APPARMOR. I don't know if that table is complete or if it 
> needs to be extended for any other MAC system.
>
> That then leads to the auparse normalizer. I don't know if we need to make 
> any changes there. You can trigger its code with ausearch --format csv or --
> format text.
>
> Also, we have some size limits in user space. How big can an event record be 
> if the file is MAX_PATH name length and it has a space in its name or 
> directory and each context is it's maximum size? We may need to think about 
> how this might change the whole userspace ecosystem's size definition, 
> MAX_AUDIT_MESSAGE_LENGTH, since this is part of the ABI. And the kernel also 
> has AUDIT_MESSAGE_TEXT_MAX. What would you get with:
>
> # /usr/sbin/auditctl -m `perl -e 'print "A"x8880'`
>
> And last...what about auditctl? Is the syscall filter going to allow filtering 
> on these other subject/object components?
>
> -Steve
>
>